Method of increasing the resistivity of a dipole suspension

ABSTRACT

An electrically responsive light-controlling device in the form of a sealed thin tanklike member having a fluid suspension of light reflective dipoles therein. Electrically conductive layers are disposed on each major side of the tanklike member to selectively impress an electrical potential across the suspension to align the dipoles. A method of increasing the resistivity of the dipole suspension is also disclosed.

ilnitd States Patent Alvin M. Marks [72] Inventor 153-16 Tenth Ave., Whitestone, NY. 11357 211 App]. No. 653,921

[22] Filed July 17, 1967 [45] Patented Dec. 7, 1971 Original application Aug. 28, 1965, Ser. No. 490,943, now Patent No. 3,341,274. Divided and this application July 17, 1967, Ser. No. 653,921

[54] METHOD OF INCREASING THE RESISTIVITY OF A DIPOLE SUSPENSION 3 Claims, 5 Drawing Figs.

[52] 11.8. C1 252/500, 23/300 [51] 1nt.Cl H011) 1/00 [50] Field of Search 350/ l 60,

a V ses [56] References Cited UNITED STATES PATENTS 2,178,996 11/1939 Land 23/300 2,440,378 4/1948 Newsome et a1. 23/300 3,281,206 10/1966 Weele 23/300 Primary Examiner Douglas J. Drummond Attorney-Albert F. Kronman ABSTRACT: An electrically responsive light-controlling device in the form ofa sealed thin tanklike member having a fluid suspension of light reflective dipoles therein. Electrically conductive layers are disposed on each major side of the tanklike member to selectively impress an electrical potential across the suspension to align the dipoles. A method of increasing the resistivity of the dipole suspension is also disclosed.

METHOD OF INCREASING THE RESISTIVITY OF A DIPOLE SUSPENSION This invention relates to electrically responsive light-controlling devices and specifically to structures which may reflect, transmit or absorb light upon the application of electrical potential thereto. This application is a division of an application filed Sept. 28, 1965 entitled Electrically Responsive Light-Controlling Devices, in the name of Alvin M. Marks, Ser. No. 490,943 now U.S. Pat. No. 3,341,274.

It is often desired to control the effect of light or other radiations upon a given surface. Thus, for example, if it is necessary to maintain a low temperature beneath a surface such as a roof, it may be important to reflect sunlight from the surface. At other times, the same surface may be used to absorb sunlight in order to heat the area beneath the roof. Such structures are valuable to implement summer and winter heating and cooling systems.

In the field of space satellites, it may be required that the surface of the satellites be absorbing when the satellite is in the shadow of the earth or the moon in order for it to absorb the long wave infrared radiation. However, when the same satellite is subjected to the rays of the sun it may be necessary to reflect the said rays in order to avoid excessive temperatures within the satellite.

Under other circumstances, it may be desirable to provide a structure which can change from light reflecting to absorbing or transmitting states with great rapidity. Such operations are useful for signaling or display uses.

Accordingly, it is an object of the present invention to provide electrically responsive light-controlling structures capable of being changed from a light reflecting to an absorbing medium.

Another object of the present invention is to provide a device which will reflect or transmit light incident thereon in response to an electrical voltage applied to the structures.

A further object of the present invention is to provide an electrically responsive light-controlling structure which can change the color of the reflected light coming therefrom in response to an electrical impulse.

An object of the present invention is to provide a surface which will increase or decrease the transmission of radiant heat impinging thereon.

A feature of the present invention is its use of minute flakes or elongated flat particles hereinafter referred to as dipoles carried in suspension which are capable of reflecting or permitting the passage of light therethrough, as the result of their alignment by an electric field or their disorientation in the absence of an electrical field.

Another feature of the present invention is its use of transparent conductive coatings which permit the dipoles to be oriented rapidly.

A further feature of the present invention is its use of a protective barrier for the conductive coatings to prevent the dipole particles or the suspending liquid from reacting with the said coatings.

Still another feature of the present invention is its use of thin transparent conducting layers on transparent supports which are spaced apart only a short distance whereby substantially complete alignment may be achieved using electric potential differences in the IQ to 500 volt range, the speed of alignment increasing with the applied voltage.

A feature of the present invention is its use of a grid or network of thin conductive lines to control the dipoles.

Another feature of the present invention is its use of external sensing devices to change the light-controlling structure from reflecting to absorbing and vice versa as required.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a fragmentary view on an enlarged scale of an electrically responsive light-controlling structure made in accordance with the present invention showing the dipole particles in a disoriented, reflecting state,

FIG. 2 is a view similar to FIG. 1, showing the aligned dipole particles,

FIG. 3 is a fragmentary view similar to FIGS. 1 and 2, showing a protective coating between the conductive coating and the dipole suspension,

FIG. 4 is a fragmentary view on an enlarged scale of an electrically responsive light-controlling structure having a plurality of layers of dipole suspensions, a further embodiment of the present invention.

FIG. 5 is a fragmentary view on an enlarged scale of an electrically responsive light-controlling structure having a plurality of conductive lines on each side of the dipole suspension for controlling dipole movement, another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing and particularly to FIG. I, 10 indicates a transparent sheet of glass, plastic or the like. A second sheet of material ll, made of glass, plastic or other fluid impervious material is spaced from the first sheet 10. A fluidtight gasket 12 is disposed between the sheets 10 and I1. adjacent the edges thereof to form a small flat tanklike area 13 between the said sheets 10 and 11. The surfaces of the sheets 10 and II which define the tanklike area 13 are covered with an electrically conductive transparent coating 14 hereinafter more fully described. The conductive coatings 14 are connected to suitable metallic strips or bus bars 15 which are disposed along the edges of the sheets 10 and 11. Electrical leads 16 are in turn connected to the bus bars 15 and lead to a suitable source of electrical potential, (not shown).

The tanklike space I3 between the sheets I0 and I1 is filled with a fluid 17 in which there is carried a suspension of particles hereinafter referred to as dipole particles 18.

When the dipole particles 18 are free to move about within the tanklike space 13 they respond to Brownian movement and become randomized as best shown in FIG. I. Since the dipole particles within the tanklike area 13 are highly reflective, needlelike or flat elongated needlelike or flat flakelike, particles, light, indicated by the arrows 19, will be reflected and emerge from the structure as beam 20. The specific nature of the reflective dipole particles 18 will be hereinafter more fully set forth.

When an electric field is imposed across the conductive coatings 14 by the application of electrical potential to the leads 16, the dipole particles 18 will become aligned with their long direction parallel to the electric field and normal to the surface of the sheets 10, 11, as shown in FIG. 2. Since the thickness of the dipole particles 18 is small compared to their length, the light 19 will be able to pass between them and reach the second sheet 11. If the second sheet II is transparent the light 19 will traverse the assembly as indicated by the emergent ray 21. If the second sheet 11 is light absorbing, it will absorb the light beam 19 within the structure of the device. It is within the purview of the present invention to make the second sheet 11 of a colored material whereby the light beam 19 will be reflected from the colored surface of the second sheet 11 and emerge as a colored beam having a hue different from the incident beam.

When the electric field is decreased or reduced to zero, the dipole particles 18 again become randomized by Brownian motion presenting many layers with their long dimension at an angle to the light rays 19. Because the dipole particles have a higher index of refraction than the suspending liquid 17 many reflections occur which reflect the incident light back in a more or less diffused pattern.

The effect of Brownian motion can be ascribed to an average relaxing force acting on the suspended particles due to thermal energy. When the electrical forces align the dipoles, the speed of alignment is expected to obey similar laws, but since the electrical forces applied are much greater than those due to thermal energy, much more rapid alignment can be expected with the applied electric force.

It will be apparent that the optical characteristics of the assembly may be varied from highly reflective to highly absorbent and also may be employed to change from highly reflective to light transmitting.

Referring to FIG. 3 it will be seen that the conductive coatings I4 are covered by a transparent protective layer 22 which is disposed upon the coatings 14 on their dipole suspension sides. The protective coating 22 is necessary where certain dipole suspensions may be chemically reactive with the conductive coatings. The protective layer 22 may, for example, consist of transparent silicon monoxide layers.

While the flat tanklike area 13 has been shown with substantial thickness in the figures it is to be understood that the drawing is merely for the purpose of clarity and in actual prac-, tice the sheets and 11 may be spaced apart, for example, a distance of from 0.05 to 0.50 millimeter. As a result of the small spacing between the sheets it is possible to get a substantially complete alignment of the dipole particles within the tanklike area 13 with voltages of the order of 10 to 500 volts.

FIG. 4 illustrates still another embodiment of the present invention in which several cells are arranged to provide a series of flat tanklike areas 13 having dipole suspensions 18. In this embodiment the dipoles may be differently colored in each of the tanklike areas so that light traversing the first tanklike area 13 may be reflected by the dipoles in the second and light traversing the first and second tanklike areas 13 may be reflected from the dipoles in the third.

The amount and extent of light penetrating the various cells in the embodiment shown in FIG. 4 can be controlled by controlling the voltage across the individual cells. Thus, if an electrical potential is applied only to the first layer of dipole suspension 18, the light will traverse the said first layer and be reflected from the second. If an electrical potential is applied to the first and second layer the light will traverse both layers and be reflected from the third layer. In this manner, three individual colored reflections may be achieved by the structure illustrated.

In lieu of conductive coatings 14 shown in FIGS. 1-4, there may be employed a network or spaced grid of parallel conducting lines 25, 25' on each sheet and disposed at right angles to each other as shown in FIG. 5. The conducting lines 25 are shown greatly enlarged for the sake of clarity. It is to be understood that the width of the lines 25 must be comparatively negligible compared to the distance between them so that light passing through the sheets 10 and 11 is not appreciably scattered or impeded by the lines 25.

The lines 25 may comprise thin rulings on the sheet surfaces into which there is worked or deposited a metallic material such as silver. The diameter of said lines must be of such order that the resistance to the flow of electrical energy is comparatively'low to avoid the effect of attenuation. The result of this construction will be that a potential may also be applied to a single pair of lines 25-25 of the network at the edge of a sheet 10, 11, so that it is substantially instantaneously transmitted to only the small area where they cross and thus only this small area is aligned. Such a structure is useful to display information such as a letter or design.

In other respects the structure shown in FIG. 5 is identical to that shown in FIGS. 1-4, in that it contains a tanklike area 13 within which there is carried a suspension of dipole particles 18.

It is within the purview of the present invention to connect the conductive lines 25-25 to separate leads in groups so that small areas within the tanklike cell 13 can be oriented while allowing the remainder of the cell to be reflective. In this manner, novel displays of light can be achieved.

DIPOLES The flat tanklike areas 13 are filled with a suitable liquid or fluid 17 in which is contained a suspension of elongated dipole particles 18. In addition to being reflective the dipole particles must also be capable of being oriented by an electrical field. Where there is to be little or no scatter of light, the length of the dipole particles is preferably of the order of 2,000 to 3,500 Angstroms or any even integral multiple thereof and their cross-sectional width of the order of 200 to 700 Angstroms. These dimensions are necessary in order that the dipole particles efiectively reflect or scatter visible light incident upon them at an angle of approximately plus or minus 40 to the normal of their longitudinal axis and add substantially no reflectivity or scatter at other angles. The extremely small width of the dipole particles 18 (of the order of 1/10 the wavelength of blue light) enables a light beam I9 projected approximately normal to the plane of the sheets 10 and 11, to traverse the dipole suspension without substantial scattering or reflection from the dipole particles 18.

Examples of particles having a suitable length to avoid light scatter when oriented within the required range for use as dipole particles are certain proteins such as the well-known tobacco mosaic virus, which has a length of between 2,800-3,300 Angstrom units, and a diameter of about I50 Angstrom units. Other suitable materials are, for example, sodium thymonucleate, myosin, guanidinium halides, etc. The size of the particle may be controlled by controlling the aggregation or crystallization of these substances.

Larger particles, up to about 50 microns in diameter by about l-micron thick may be employed where light scatter is not objectionable; as with variable reflectivity panels where image transmission is not a problem.

Colloidal suspensions of anisotropic, birefringent elongated dipole particles of substances which have an inherently large dipole moment, may also be used. These colloidal particles may preferably be suspensions of crystallites in a suitable liquid. The crystallites may be obtained from a widely dispersed class of organic or inorganic chemical compounds; for example, meconic acid, quinine sulfate, certain protein crystallites previously set forth, quartz, etc.

Mixtures of particles having a substantially constant reflectivity through the visible spectrum are recommended as dipole particles. Such mixtures may comprise, for example, particles such as vanadium pentoxide, tungstic acid, or herapathite sols preferably in liquid suspensions.

Herapathite crystals suitable for use as absorptive dipole particles may be prepared in accordance with the following:

Preparation of Herapathite Crystals Solution 1 Iodine 1 gram Normal Propanol 4 grams Dissolved by continuous heating and shaking. This contains 7.89X l0 moles of I, per gram. Solution 2 Ouinonc bisulfatc l5 grams Methanol 3| grams Solution 3 Nitrocellulose solution 5 grams Methanol 5 grams Solution 4 Solution 3 5 grams Solution 2 1 gram This solution becomes milk white and will clear temporarily if heated. The addition of solvents will clear the solution permanently. The cleared solution is solution 5.

Solution 5 Solution 4 1 gram Methanol 0. l 77 gram Butyl acetate 0.193 gram Solution 5 is then warmed and pressure filtered to remove any centers of crystallization. This mixture now contains 7.25Xl0 moles of quinine bisulfate per gram. The solution may gel on standing, but may be broken by warming and shakmg.

Solution 6 To 5 grams of cooled solution 5, (quinine bisulfate in nitrocellulose) add 0.75 grams of cooled solution I (iodine). The solution is now blue-black due to the formation of a suspension of crystals of quinine trisulfate dihydroiodide tetraiodide hexahydrate (4C H O N 3H SO', ZHI'I '6H O). This substance is called Herapathite.

Another form of dipole particle suitable for the present invention may comprise anisotropic crystal needles coated with metallic films of silver, platinum or the like.

In lieu of crystal needles there may also be employed flakelike particles such as glass flakes or natural and synthetic pearlescence, for example, bismuth or lead carbonate crystals. These crystals are hexagonal platelets about 20 microns in diameter by about 0.5-microns thick, and of an index of refraction of about 2.09.

Amorphous glass flakes having an index of refraction of 2.0 or more and preferably in the range of 2.2 to 2.6 and a length of 0.5 to 20 microns and a thickness of 0.05 to 2 microns are suitable for use as flake dipole particles. Glass flakes of this nature have been aligned for the purpose of permitting light to pass therethrough and are also highly reflective when disoriented. Ordinary commercially available glass flakes having an index of about 1.52, may be coated with titanium dioxide films on one or both surfaces in known manner and will then have an index of refraction of about 2.7. A lower concentration of these will provide high reflectivity.

Increasing the Brownian motion of the dipole particles 18 will facilitate the speed of disorientation. In addition to mechanical disorientation hereinabove described, radiating the entire tanklike area 13, with infrared radiation and providing a suspending liquid capable of absorbing said radiation will greatly facilitate the disorientation of the dipole particles 18 when the electrical field is removed.

LIQUIDS Suitable liquids for the dipole suspensions may comprise certain well-known plasticizers such as the silicone fluids, dibutyl phthalate, diphenyl and others. In some instances water or certain of the higher alcohols may be used as the suspending fluid.

The viscosity of the liquid is a factor in the speed of response. The viscosity depends on the suspending fluid used, the concentration of the colloidal medium and the dipole particles or crystals. When low-viscosity fluids are used the viscosity of the dipole suspension will decrease, particularly at greater dilutions.

In many solutions the specific resistance of the solution is too low due to the presence of ionizable liquids such as water or methanol or dissolved electrolytes. Power is therefore dissipated between the electrodes and a bubbling action inter feres with the alignment of the dipole particles or crystals. This problem may be overcome by adding high-boiling solvents having a boiling point in excess of 150 C. and a freezing point below C. Evaporating techniques are then utilized to extract the low-boiling solvents and thereby increase the resistivity of the solution. For example, by the addition, of equal parts of dibutyl phthalate as the plasticizer in the above solution 6 and boiling off the methanol by applying vacuum at room temperature, it was found that the specific resistance was 47 times lower than prior to the evaporation of the lighter solvents.

ln carrying out this technique using the Herapathite crystals referred to above, 5.75 grams of the desired high-boiling TRANSPARENT CONDUCTIVE COATINGS Transparent conductive coatings as hereinabove described suitable for use on the surfaces of the sheets 10 and Il, may comprise stannic oxide films such as are sold by the Liberty Mirror Company under the designation EL-SX.

It IS within the purview of the present invention to cover the grid or network of conductive lines 25 with a conductive coating such as is herein disclosed. Such coating will facilitate the uniform distribution of the charge over the surface of the sheets 10 and 11. The conductive coatings may also be overlain by a thin transparent layer such as a glass sheet for protective purposes.

REFLECTING FILMS ON DIPOLES The elongated dipole particles I8 may be formed from an anisotropic crystal needle upon which is deposited a metal reflecting film of silver, platinum or some other light-reflecting material. This can be accomplished by adding to the suspended crystals a suitable deposition liquid similar to that which is used for the deposition of silver on glass. The deposition liquid may also contain coloring matter to give the reflecting surfaces a color tone as desired.

Alternatively, a high index coating such as titanium dioxide may be applied as previously set forth herein.

From the foregoing it will be apparent that there have been provided electrically responsive light-controlling devices capable of transmitting or reflecting either partially or wholly light incident thereon. In addition these devices change the color of light reflected therefrom to one or more colors as desired. The devices are rapid in action and will not break down over continued use.

Having thus fully described the invention what is claimed as new and desired to be secured by Letters Patent of the United States, is:

I. The method of increasing the resistivity of a dipole suspension comprising the steps of adding a high-boiling highresistivity solvent having a boiling point in excess of C. to the low-boiling low-resistivity suspending solution and thereafter evaporating the low-boiling solvent from the solution.

2. The method according to claim I in which the dipole suspension contains Herapathite crystals.

3. The method according to claim 1 in which the high-boiling solvent is a plasticizer selected from the group consisting of dibutyl phthalate, di iso octyl adipate and dibutyl sebacate. 

2. The method according to claim 1 in which the dipole suspension contains Herapathite crystals.
 3. The method according to claim 1 in which the high-boiling solvent is a plasticizer selected from the group consisting of dibutyl phthalate, di iso octyl adipate and dibutyl sebacate. 